1. Field of the Invention
The present invention relates to a feed screw mechanism in which an internally threaded member fixed within an output rod is threadedly engaged with an externally threaded member, and one axial end of said externally threaded member is connected to a drive source, so that said output rod is moved relative to said externally threaded member by rotating said externally threaded member relative to said internally threaded member.
2. Description of the Related Art
There is a system known from Japanese Patent Publication No. 6-47388, in which an upper link and a lower link of a suspension device of a vehicle are controlled for expansion and contraction by an actuator, thereby inhibiting variations in camber angle and ground-contact tread with the bumping and rebounding of a wheel to enhance a stabilized steering performance, wherein the actuator is comprised of a feed screw mechanism including an externally threaded member and an internally threaded member which are adapted to be rotated relative to each other by a motor.
In such feed screw mechanism, in many cases, the externally threaded member is formed of iron, and the internally threaded member is formed of bronze or aluminum. A sufficient strength is required for an output rod of the feed screw mechanism and for this reason, it is impossible to form the output rod integrally with the internally threaded member formed of bronze or aluminum. Therefore, it is required that the internally threaded member is accommodated within a hollow output formed of iron to ensure a strength and is integrally fixed to the output rod by a fixing means such as a locking nut.
In such a case, as described in “DETAILED DESCRIPTION OF PREFERRED EMBODIMENT” in this specification, when the conventional actuator has been driven for contraction, a pulling load is applied to the externally threaded member, while a compression load is applied to the internally threaded member. This causes a problem that the transmitted load (the contact surface pressure) on the tooth flanks of the externally threaded member and the internally threaded member is non-uniform in a direction of an axis of the feed screw mechanism, which exerts an adverse influence to the durability and the smooth operation of the feed screw mechanism.